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Intersil Corporation 2000
HC5503PRC SLIC and the Texas Instruments
TP3057A Combined PCM CODEC and Filter
Reference Design using the HC5503PRC
SLIC and the Texas Instruments TP3057A
Combined PCM CODEC and Filter
The network requirements of many countries require the
Analog Subscriber Line Circuit (SLIC) to terminate the
subscriber line with an impedance for voiceband frequencies
which is complex, rather than resistive (e.g., 600
)
. This
requires that the physical resistance that is situated between
the SLIC and the subscriber line, comprised of protection
and/or sensing resistors, and the output resistance of the
SLIC itself, be adapted to present an impedance to the
subscriber line that varies with frequency. This is
accomplished using feedback around the SLIC.
The purpose of this application note is to show a means of
accomplishing this task for the HC5503PRC and Texas
Instruments TP3057A Combo.
Discussed in this application note is the following:
2-wire 600
impedance matching
2-wire complex impedance matching
Receive gain (4-wire to 2-wire) and transmit gain
(2-wire to 4-wire) calculations
Transhybrid balance calculations
Reference design for 600
2-wire load
Reference design for China complex 2-wire load
Impedance Matching
Impedance matching of the HC5503PRC to the subscriber
load is important for optimization of 2 wire return loss, which
in turn cuts down on echoes in the end to end voice
communication path. It is also important for maintaining
voice signal levels on long loops. Consider the equivalent
circuit shown in Figure 1.
The circuitry inside the dotted box is representative of the
SLIC feed and transmit amplifiers. The feed and transmit
amplifiers pass the voice signals in the receive and transmit
directions respectively. Without the feedback block f(Z
0
), the
termination resistance at V
2W
would equal the two protection
resistors (R
P
) and the two sense resistors (R
S
), as the feed
amplifiers present a very low output impedance to the
subscriber line. The desired termination impedance at V
2W
is
Z
0
. The feedback block f(Z
0
) matches the SLICs output
impedance (Z
SLIC
) plus the two protection resistors (R
P
) and
the two sense resistors (R
S
) to the load (Z
0
).
Impedance matching of the HC5503PRC is accomplished by
making the SLIC’s impedance (Z
SLIC
, Figure 2) equal to the
desired terminating impedance Z
0
, minus the value of the
protection and sense resistors. The desired impedance at the
input to the SLIC is given in Equation 1.
FIGURE 1. IMPEDANCE MATCHING BLOCK DIAGRAM
FEED
AMPS
TRANSMIT
+
-
2*Rp
2*Rs
f(Z
0
)
TO
FROM
COMBO
V
IN
x2
x2
V
TX
COMBO
Vs.
V
RX
V
2W
-
+
Z
O
Z
SLIC
Z
O
(EQ. 1)
Z
SLIC
Z
0
2
R
P
×
–
2
R
S
×
–
=
FIGURE 2. IMPEDANCE MATCHING
T
R
INTERSIL
HC5503PRC
R
X
T
X
0.47
μ
R
f
0.47
μ
R
a3
= 2R
f
R*4R
S
R
P
+R
S
R
S
R
P
R
S
R
P
Z
O
I
L
V
2W
-
VFRO
R
a2
= 4R
s
R
Z01
= Z
0
TEXAS
GSX
V
RX
-
V
S
R
a4
= 2R
f
V
GSX
R
a1
=
Z
SLIC
Z
O
V
IN
V
TR
+
-
V
TX =
4R
S
I
L
+
VFXI-
VFXI+
DR
DX
+ 4dB
- 4dB
-
INSTRUMENTS
TP3057A
T
F
R
F
Application Note
September 2000
Authors: Don LaFontaine, Chris Ludeman